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fifth wire

My HVAC thermostat is 4 wire system W,R,Y,G. I just bought a new thermostat that uses 5 wires, w/extra Common connection. I was considering running a 5th wire, no easy task. then i removed old thermostat and pulled the wires out of the wall and there is a 5th black wire that was not used. So, i go to the furnace in the garage and look at the control board and there are the same four wires hooked up to the board and the fifth black wire not hooked up. So, all i need to do is connect this wire to common or furnace ground, right?

Before i did that, I checked several things.

At thermostat end of the wires with the thermostat disconnected I get 28 volts AC between R and each of the other three, but between R and the black I get volts starting at 7, 8, or 9 volts then it climbs up rapidly at 1st then slowly and by the time it gets to 12.x it is increasing very slowly. Now i go to the furnace and I get the same 28 volts between R and each of the others, except black. Between R and Black I get zero.zero volts. Between the black and ground is also zero.zero volts.

Most thermostats can be used with systems that have a different number of wires. Check the user guide that came with the thermostat it's likely it can be used with several different system types and numbers of wires. I changed out the thermostats at the church I attended a few years ago for programmable thermostats. In the back of the user manual there were wiring diagrams for many systems and numbers of wires. I'm not a HVAC technician but I'd bet if you hooked up the new thermostat with the existing wires in use to the appropriate letter that it would work fine.